One of the major test objectives of Dragon’s Commercial Orbital Transportation Services (COTS) test flight was completed early on Thursday, as the SpaceX spacecraft flew under the International Space Station (ISS) and successfully received commands via a UHF system. Dragon continues to be on schedule for its historic berthing with the ISS on Friday.

See Also

“The SpaceX Far Field demos completed were the Abort Burns, Absolute Global Positioning System (AGPS) checkout, and Free Drift Recovery. Dragon arrives at the 2.5 km co-elliptic orbit for the fly-under and associated Fly Under Demos at approximately 2:49am CDT on Thursday.”

Chasing down the Station, Dragon conducted what is know as far field phasing operations, ahead of Thursday’s major objective of flying under the ISS for a test of the UHF communication ability, via the COTS UHF Communication Unit (CUCU) system, allowing for commands to be received from the Station.

The Commercial Orbital Transportation Services (COTS) Ultra High Frequency (UHF) Communication Unit rode in the middeck stowage locker on Atlantis during STS-129 late in 2009, before being handed over to ISS crewmembers ahead of the demonstration flights.

The new system allows ISS crewmembers to monitor and command approaching or departing Dragon spacecraft during cargo delivery missions to the orbiting laboratory.

“CUCU provides a bi-directional, half-duplex communications link between Dragon and ISS using existing ISS UHF Space to Space Station Radio (SSSR) antennas, which provides a communication path between MCCX (SpaceX) and Dragon during proximity operations and a command security between ISS and Dragon,” noted STS-129 FRR documentation on L2.

The on-orbit checkout of the CUCU began January 25, 2010, when astronaut Jeff Williams, ISS Expedition 22 Commander, worked with ground-based team members at SpaceX headquarters and ISS mission control in Houston to power-up and checkout the new system.

On March 11 of that year, SpaceX and Mission Control Houston performed an additional series of tests, using the new system to send communications between the ISS and the NASA Dryden ground station.

This provided a baseline of the radio frequency performance and confirmed the first set of antennas performed as expected and is ready for mission operations.

The tests employed live video and telemetry links from the ISS to verify the hardware’s functionality, broadcast and reception signal strengths, and the system’s stability over long-duration operations.

“The success of this series of tests speaks to our close collaboration with NASA as well as the SpaceX process that allowed the rapid development of this new hardware,” said Marco Villa, SpaceX Mission Operations Manager, at the time of the tests.

For Thursday’s test, Dragon conducted a High Adjustment (HA2) burn, prior to Co-Elliptical burn (CE2) on Thursday. Both burns were successful, allowing for the spacecraft to carry out the fly under of the Station, riding on a “racetrack” circuit under, in front, over and behind the ISS.

“Dragon flies under ISS at 2.5 km. During the fly-under UHF communication via the CUCU system is established with ISS. The Commanding from ISS and RGPS Demonstrations are performed. 2.5 km was specifically chosen by SpaceX because Dragon does not pose a hazard at that distance,” noted the L2 mission walkthrough – L2 Link.

The main objective of the test was to send a command from the Station to the Dragon, confirmed by flashing a strobe light on the SpaceX vehicle, as the spacecraft flew into ISS’ UHF comm range.

With the DragonEye LIDAR acquiring the ISS, the ISS then successfully set up the CUCU link and commanded the strobe light as planned. While lighting conditions did not allow for the ISS crew to observe the strobe light, ISS cameras did manage to gain some views of the Dragon, as it flew under the station.

Dragon then proceeded forward of the ISS and is now in the process of “reversing” the CE and HA burns ops to fall back behind to the starting point of the invisible racetrack ahead of Friday’s return beneath the ISS, in readiness for berthing.

“The ISS crew will briefly interact with Dragon, monitoring the fly-under and sending a strobe command from the CCP,” added the walkthrough. “The 2.5 km fly-under is completed and a pre-planned re-rendezvous is initiated. Go/No-Gos for the forward and rear re-rendezvous burns are required. The re-rendezvous ends with Dragon on a co-elliptic 10 km below ISS.”

Establishing communications between the Station and Dragon is critical, ahead of Friday’s key objective of carefully allowing Dragon to approach the ISS, via a series of Go/No Go points.